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REFRACTORIES                                         491
the well-defined crystalline state to that of a typical colloid. From the technical standpoint, roughly, three classes of refractory clays may be distinguished, viz., kaolins, flint clays, and plastic clays.
Kaolins.The first class of materials, usually of geologically primary origin, consists, in the purified state, of white clayey matter, containing both the crystalline and amorphous varieties of clay base. In some of these clays the crystalline constituents predominate, as in the North Carolina kaolins. The plasticity of these clays is but feebly developed, though where the granular matter has been broken, down by the action of water or other agencies to the amorphous condition, a fair working quality may result. These clays, on account of their whiteness, are used in the pottery industries.
There are, however, kaolins which possess a fair degree of plasticity, as the Georgia and Florida kaolins and some of the English china clays. These, as long as they maintain good whiteness, are highly valued in the manufacture of white ware and porcelain products. Frequently, however, increased plasticity is coincident with increased content of fluxes and consequent reduction in refractoriness. While marked plasticity in itself, of course, does not mean reduced refractoriness, it indicates geological conditions which tend to incorporate impurities in the clay.
Owing to their purity (absence of basic oxides) the kaolins are the most refractory clays. Thus a well-known kaolin from Zettlitz, Bohemia, which is considered a standard material of its kind, shows a softening temperature corresponding to Seger cone No. 35, estimated to correspond to a temperature of 1,750C., according to the scale adopted by the Bureau of Standards (palladium = 1,549C.; platinum = 1,755C.)-
Flint Clays.The so-called flint clays embrace many materials of a grade of purity corresponding closely in composition to the best grade of kaolins. Like the latter, they may, of course, deteriorate into clays of comparatively low refractory value. As has been said, their composition is very close to that of the kaolins, even as to the content of chemical water. Physically they are unlike the soft and chalky kaolins in possessing a hard, dense amorphous structure, showing a peculiar well-defined conchoidal fracture. The color is usually gray. The initial plasticity is exceedingly feeble, though if exposed to the weather or if ground either dry or wet the condition of colloidal "set" may be partially overcome and sufficient plasticity developed for molding purposes. Owing to the weak plasticity possessed by flint clays, their drying shrinkage when ground and made up with water is very slight. On the other hand, in burning these clays undergo a considerable shrinkage. In this respect, however, considerable difference exists between the various materials, apparently dependent upon the colloidal state of the clay base. Thus, for instance, a Pennsylvania flint clay from Cloarfield County showed a linear shrinkage of 5.5 per cent up to cone 20; a Maryland clay of the same type was found to possess a maximum shrinkage of 5.6 per cent at cone 11, which decreased on further heating to 3.5 per cent at cone 20, indicating an increase in volume; a Kentucky flint clay, on the other hand, showed a steady gain in shrinkage up to cone 9, when it reached a linear contraction of 9 per cent, while at cone 20 it showed 9.5 per cent.1
The volume shrinkage characteristic of these clays subjects the structure of the product into which they enter to a severe strain, which, owing to the low tensile strength, may cause serious difficulty due to cracking and checking, so that it may be
i J. M. KNOTE, Trans. Am. Ceram. Soc., Vol. 12.